Chemical strengthening of glass, also called ion-exchange strengthening or chemical tempering, is a technique to strengthen a prepared glass article by increasing compression within the glass itself. It generally involves introducing larger alkali ions into the glass chemical structure to replace smaller alkali ions already present in the structure. A common implementation of chemical strengthening in glass occurs through the exchange of sodium ions, having a relatively smaller ionic radius, with potassium ions, having a relatively larger ionic radius by submerging a glass substrate containing the sodium ions in a bath containing molten potassium salts.
Chemical strengthening is often utilized to increase compression in glass. Increased compression within the glass of a glass part is associated with increased strength, increased abrasion resistance and/or increased thermal shock resistance in the glass. The increased compression can be introduced to various depths in glass and is often implemented within a glass surface layer. Chemical strengthening is commonly utilized for treating flat glass. But it may also be used for treating non-flat glass articles, such as cylinders and other shapes of greater geometric complexity.
Flat glass is commonly manufactured by a number of known techniques. These include the float glass method and drawing methods, such as the fusion down-draw method and the slot draw method. However, a prepared flat glass article may have variations in its chemical composition and/or structure at different locations in the glass. For example, flat glass that is manufactured by the float glass technique is often prepared by spreading softened glass material on a molten metal surface such as tin. The glass is then cooled to form a solid, flat glass. As a result, the prepared flat glass often contains a greater amount of tin on the side that was nearer the molten tin and the concentration of tin is commonly greater near the surface of that side.
Chemical strengthening is often used to treat glass having variations in chemical composition and/or structure at different locations in the glass. The variations produce locations that are treatment-rich or treatment-poor relative to each other for ion exchange and/or compression development in chemical strengthening. When chemical strengthening is used to treat such glass, the introduced compressive stress is often not uniformly distributed.
When compressive stress is not evenly distributed in chemically strengthened glass, this may introduce a bending moment and, subsequently, an induced curvature in the glass treated by chemical strengthening. The effect of introducing induced curvature by chemical strengthening is particularly apparent for glass articles having a smaller width. This is commonly problematic in flat glass having a smaller width, such as less than 25 millimeters, as the bending moment in thinner glass introduces greater curvature from chemical strengthening. Float glass substrates having a width of 2.0 mm or less, and particularly those having a width of 1.0 mm or less, often suffer from having highly significant curvature introduced through chemical strengthening.
Induced curvature is often undesirable and is often especially problematic in manufacturing thin flat glass articles according to manufacturing specifications that call for the enhanced physical properties associated with chemical strengthening, but without significant induced curvature. For example, glass used in many manufactured electronic articles, such as displays for “smart” phones, often requires a display glass that is substantially flat and high in strength and in abrasion resistance.
Chemically strengthening a thin, flat glass substrate, such as an article having two major surfaces and variations in chemical structure or composition within the glass, is often associated with a non-equivalence of interdiffusion of invading alkali ions and/or compression generation properties between the major surfaces of the substrate. The effect is that local forces in the glass about the mid-plane, associated with distance from the mid-plane of the glass article, are not equivalent when compared as these occur from the treatment-poor surface to the mid-plane and from the treatment-rich surface to the mid-plane. Thus the net bending moment about the mid-plane in the glass is non-zero (i.e., there is a non-zero net bending moment of the stress about the mid-plane). As a result, bending stresses are generated.
For glass articles having a thin cross-section, bending stresses from chemical strengthening often generate deflection from flat in the glass article. This is especially common in glass made by a float glass method. But deflection is also generated in glasses made by other methods that have variations in chemical structure or composition within the glass. Thin glass manufactured using a float glass process often exhibits measurable curvature after chemical strengthening. The direction of curvature is often concave on a surface that is “poorer” in allowing interdiffusion of invading alkali ions and convex on a surface that is “richer” in allowing interdiffusion of invading alkali ions.
In recent years, various types of efforts have been made attempting to overcome the problem of induced curvature associated with the chemical strengthening of glass. One approach involves grinding and polishing a glass substrate prior to chemical strengthening. The grinding and polishing is performed to remove those parts of a glass having a different chemical composition and/or structure. An example of this approach is grinding and polishing a flat glass made by the float method to remove the surface layer(s) containing a significant amount of tin. However, grinding and polishing the float glass often introduces abrasions and may introduce other physical defects. These defects are compounded by the added time and expense associated with performing the grinding and polishing.
Other approaches have involved secondary chemical treatments of prepared glass done prior to chemical strengthening. The secondary chemical treatments are utilized in an attempt to address differences in chemical composition and/or structure at different locations in the glass. However secondary chemical treatments can alter the physical properties of the glass and otherwise degrade a glass produced through subsequent chemical strengthening. Also, like grinding and polishing, secondary chemical treatments involve the time and expense of extra processing done prior to chemical strengthening.
Given the foregoing, chemically strengthened glass and methods for making chemically strengthened glass are desired in which the strengthened glass has reduced induced curvature. It is also desired that the strengthened glass not have the drawbacks associated with grinding and polishing or secondary chemical treatment(s) applied in prior methods associated with the chemical strengthening of the glass. It is also desired that the strengthened glass having reduced induced curvature also have the improved physical properties of chemically strengthened glass, such as higher strength, higher abrasion resistance, and/or higher thermal shock resistance.